Conventionally, when breeding and rearing aquarium fish such as goldfish, tropical fish, or marine fish inside a water tank, the water tank is provided with an internal or external filter device equipped with an air pump or a submersible pump, and water-tank water inside the water tank is filtered by forcibly circulating it to the filter device by driving the pump, but since it is impossible to obtain water quality that is necessary and sufficient for breeding and rearing aquarium fish just by filtration with such a filter device, it is necessary to regularly change water-tank water inside the water tank. A cleaner for the inside of a water tank in which water is changed by sucking up water-tank water together with waste material such as fish excrement or leftover food is conventionally known (ref. e.g. Patent Publication 1 below). Patent Publication 1: Japanese Patent Application Laid-open No. 2000-209978